1. Field of the Invention
The present invention relates generally to photolithography for integrated circuits and more specifically to a method of providing levelling and focusing adjustments on a semiconductor wafer and a photolithography apparatus using the method.
2. Description of the Related Art
In a known photolithography process, a chip pattern is printed in each of a plurality of rectangular cells defined on a semiconductor wafer. Since the wafer is of a circular shape, those cells located at or near the periphery of the wafer may be left unexposed to the light of chip pattern. However, if a positive photoresist is applied to the wafer, the unexposed portion of the photoresist survives after the exposed portion is etched away by a solution. For this reason, the peripheral region of the wafer that is coated with a positive photoresist is also exposed to the chip pattern. However, due to the inherent curvature of the wafer near its circumference, the light modulated by the chip pattern is not precisely focused on the peripheral cells, causing some of the peripheral region of the positive photoresist to be left over as undesired remnant material.
According to another photolithography process, several chip patterns are printed on a single mask (or reticle) and the wafer is exposed to all chip patterns of the mask. According to this technique, only those cells where chip patterns are appropriately printed are shipped as marketable products and others are discarded. However, the problem is that the peripheral cells are not exposed to precisely focused light.
Therefore, a need does exist to precisely control the surface of a wafer at all cell locations for appropriate levelling and focusing purposes.